Topic 6: Chemical Kinetics

Question 1

reaction rate

Answer 1

rate of change in a concentration (change in concentration per time)

• increase in concentration of products per unit time (or decrease in concentration of reactants per unit time)

Question 2

reaction rate units

Answer 2

mol dm^-3 s^-1

Question 3

when is the reaction rate greatst?

Answer 3

• at the start of the reaction and decreases as the reaction proceeds
• not constant during reaction

Question 4

how can rate of reaction be measured?

Answer 4

1. change in volume of gas produced
2. change in mass
3. change in transmission of light colorimetry/spectrophotometry
4. change in concentration measured using titraion
5. chnage in concentration measured using conductivity
6. non-continious methods of detecting change during a reaction; ‘clock reactions’

Question 5

explain measurement of ROR using:

change in volume of gas produced

Answer 5

1. collecting gas and measuring chang ein volume as regular time intervals
2. use of a gas syringe
3. gas can be collected by displacement of water from inverted burette (if gas law low solubility in water)

Question 6

explain measurement of ROR using:

change in mass

Answer 6

1. if reaction gives off gas= decrease in mass can be measured by standing reaction mixture directly on balance
2. doesnt work well with hydrogen as its too light

Question 7

explain measurement of ROR using:

change in transmission of light colorimetry/spectrophotometry

Answer 7

1. good if one of the reactants/products is coloured (gives characteristic absorbtions in visible region)
2. use of indictator to genarate compounds
3. colorimeter/spectrophotometer pass light through selected wavelength though solution and measures intesnity of light transmitted by reaction components
4. allows for continious reading (graph of absorbance) `

Question 8

explain measurement of ROR using:

change in conc. using titration

Answer 8

1. use of standard solutions
2. samples drawn from reaction mix at regular time intervels and analyse by titration
3. quenching used (substance is introduced which stops reaction at th emoment its withdraw; freez frame)

Question 9

explain measurement of ROR using:

changes in conc. by conductivity

Answer 9

total eletrical conductivity; depends on conc of ions + their charges
- this changes during reacion; conductivity can be measured using conductivity meter (immersing electrodes in solution)

Question 10

explain measurement of ROR using:

‘clock reactions’

Answer 10

• use the ‘ed point’ to stop the clock
• time taken to reach this point for the same reaction under different conditions shows the different rates of reactions
• limitation; only gives average rate over time interval

Question 11

collision theory

Answer 11

1. values of kinetic energy greater than activation energy

2. appropiate collision geometry

Question 12

temperature

Answer 12

in kelvin (K) is proportional to the average kinetic energy of the particles in a substance

Question 13

heat

Answer 13

in kelvin (K) is proportional to the TOTAL kinetic energy of the particles in a substance

Question 14

maxwell boltzmann distribution

Answer 14

• distribution grapho of reaction
• measures number of particles with kinetic energy E on Y axis
• kinetic energy, E on x-axis
• area under curbe represents total number of particles in sample

Question 15

activation energy

Answer 15

minimum value of kinetic energy which particles must have before they are able to react

Question 16

factors affecting ROR

Answer 16

temperature
concentration
particle size
pressure
catalyst

Question 17

how temperature affects ROR

Answer 17

increase in temp= increase in ROR

• increase in kinetic energy= increase in collision frequency
• increase in frequency of collisions with articles with required activation energy to overcome barrier which increases the frequency of succesful collisions resulting in reaction

Question 18

how concentration affects ROR

Answer 18

• increasing the conc. of reactants increases the rate
• increasing conc.= increasing frequency of collisiosn between reactant particles= increasing frequency of succesful reactions (particles closer together)

Question 19

how patrticle size affects ROR

Answer 19

• in solids; decreasing particle size increases rate
• making solids into smaller parts; increases surface area; allows more contact and probability of collisions between reactants

Question 20

how pressure affects ROR

Answer 20

• in gases; increasing presssure increases arate of reaciton

- more pressure on gas; compreses gas; increases its concentration; increases frequency of collisions

Question 21

how a catalyst affects ROR

Answer 21

• increses ROR without itself undergoing permanent chemical change
• provide alternate route for reaction with lower activation energy
• means that large fraction of particles will now have sufficent kinetic energy greater than activation energy= able to undergo collisions

Question 22

catalyst

Answer 22

substance that icnreases the ROR without itself undergoing permanent chang

Question 23

biological catalys

Answer 23

enzymes; used in domestic and industrial processes (biotech and industry)